The mechanism of deoxyadenosylcobalamin participation in ribonucleotide reduction is being investigated by the use of electron spin resonance and optical methods. In particular information about the postulated reactive cobamide intermediate is being sought. Binding of cobamides, substrates and activators is being determined by equilibrium dialysis and other methods. The effect of one ligand on the binding of others is being given special attention. Physical and chemical properties of ribonucleoside triphosphate reductase are being studied particularly in relation to the catalytic site and allosteric properties.